1. Field of the Invention
This invention relates to an air compression hammer drill tool. More particularly, this invention relates to a downhole air compression hammer that impacts while simultaneously rotates the bit, thereby assuring maximum penetration of the bit in an earthen formation independent of the rotation of the drill string.
2. Description of the Prior Art
In percussion drilling the rock cutting mechanism is of an impacting nature rather than shearing. Therefore, the drill bit rotational parameters, e.g. torque and rpm, are not relevant from a rock formation breaking point of view, except for the necessity that the cutting elements of the bit need to be "indexed" to fresh rock formations. In straight hole air drilling, and especially in mining, this need is achieved by rotating the drill string slowly. This is accomplished in conventional hammer bit operations by incorporated longitudinal splines which key the bit body to a cylindrical sleeve at the bottom of the hammer (commonly known as the sub driver). The drill string rotation is then transferred to the hammer bit itself. Experience has proven that the bit optimum rotational speed is approximately 20 rpm for an impact frequency of 1600 bpm (beats per minute). This rotational speed translates to an angular displacement of approximately 4 to 5 degrees per impact of the bit against the rock formation. Another way to express this rotation is the cutters positioned on the outer row of the hammer bit move at the rate of one half the cutter diameter per stroke of the hammer.
An example of a typical hammer bit connected to a rotatable drill string is described in U.S. Pat. No. 4,932,483. The downhole hammer comprises a top sub and a drill bit separated by a tubular housing incorporating a piston chamber therebetween. A feed tube is mounted to the top sub and extends housing and over the feed tube. Fluid porting is provided in the feed tube and the piston to sequentially admit fluid in a first space between the piston and top sub to drive the piston towards the drill bit support and to a second space between the piston and the drill bit support to drive the piston towards the top sub.
Rotary motion is provided to the hammer assembly and drill bit by the attached drill string powered by a rotary table typically mounted on the rig platform.